Pressure switchers are a commonly used instrument in the process industry.

Like many instruments used in science, pressure switchers need to be calibrated to ensure their accuracy and reliability. The wrong calibration can cause errors in results or damage the equipment, costing a laboratory or industry significant money. Here’s a guide to what to expect when purchasing your pressure switcher.

What is a Pressure Switcher and How Does it Work?

A pressure switch is an instrument that measures pressure and has an electrical switch function programmed to operate at a specific pressure. These devices rely on water to function—the pressure presses against the sensor, which causes the switch to open and close.

The pressure that opens and closes the switch is called the set point. Which most standard pressure switches, the pressure level has a low of 30 PSI and a high of 50 PSI. This means that at 30 PSI, the switch opens and allows for water flow; at 50 PSI, the switch will close and turn off.

Normally Open (NO) and Normally Closed (NC) Switches 

Typically, open switches have the switch terminal open when no pressure is connected. When enough force is applied, the switch closes.

Ordinarily, closed switches are closed when no pressure is connected, and when enough pressure is applied, it opens.

The determining factor in which one is best for your use depends on what kind of circuit you want to drive with the switch.

Dry and Wet Switches 

A wet switch needs liquid on both high- and low-pressure ports to control the switch. These switches are in filter status, monitoring strainer baskets, chiller efficiency, and pressurized tank applications.

A dry switch uses air or non-corrosive gas on the low- and high-pressure ports. Dry switches are commonly used for measuring low pressures. However, the switches can also be used for high-pressure tests and measurement applications. Some examples are cleanrooms, isolation rooms, filter status, leak detection, and stairwell pressurization.

The dry and wet switches vary in function, but one is not necessarily better. Rather, it relies on the application it will use and the industry you work in.

Safety Pressure Switches

Safety switches are used in safety instrument systems with specific safety classifications. The calibrations of these are regulated.

These switches stay static most of the time without ever functioning. They don’t toggle open and close in normal usage; instead, the switches wait if the safety alarm level is met, then they operate.

Because these switches do not operate often, there is the risk that the systems will not work when needed.

When calibrating, do not exercise these safety switches before calibration. Instead, capture the very first point when the switch operates. Usually, the first time the switch has been turned on and is functioning, the most pressure is used versus the following times after that. For this reason, to calibrate the instrument, it must be unused for a certain amount of time. Unlike the safety switches, regular switches must be exercised a few times before calibration.

How to Calibrate Pressure Switches

When selecting a switch type, the state should be considered so that the switch status should remain safe if the power supply fails or a cable becomes loose.

In the case of a safety switch, it should be configured so that if a cable comes loose, the alarm goes on. It is designed to be fail-safe.

How to Calibrate Pressure Switches

There are a few steps when getting ready to calibrate switches.

• Preparation & Safety – If the switch is installed in the process, isolating it from the pressure line is essential. You also need to make sure to disconnect any circuit that the switch is controlling. Some switches may have primary or another dangerous voltage across the switch terminals when they open, ensuring it is isolated.
• Pressure Ramp – To calibrate a pressure switch, you need to provide a slowly changing pressure ramp, moving across the operating points of the switch. Depending on the switch type, you must supply suitable pressure to start the calibration. A few ways to provide the input pressure; a calibration hand pump, shop air supply with a precise pressure controller, or an automatic pressure calibrator can be used to do so. It is vital to provide a slow pressure ramp to see the accurate pressure whereby the switch operates. If the pressure changes too quickly, you cannot accurately capture the pressure point when the control works.
• Measuring the Switch Output – You will need a tool to measure the switch terminals. Sometimes, it may be an Ohm meter, a voltage meter, or a current meter. Some devices can program a trigger level that suits the switch and enables the status change to be captured automatically.
• Capturing the Operation Points – In the switch calibration, you need to capture the input pressure at the very moment when the output state changes. Some devices can capture the input pressure automatically at the very exact moment when the switch output changes its state. However, you can catch the input pressure manually, only with a prolonged input pressure for accuracy.
• Delayed Output – Some industrial switches may delay adding to the output not to work too quickly. You should find out if your switch is delayed, as the calibration needs to be done slower than expected.

SRP Can Help

SRP control systems Ltd offers solutions and help for pressure switch calibration and many other types of instrument calibration. Contact us today, and let us make your business run as smoothly as possible.